Implement MetaWare Hostlink library

[abrodkin]

We used to use so-called "GNU hostlink" for interaction between simulated ARC target & nSIM, see https://github.com/foss-for-synopsys-dwc-arc-processors/newlib/blob/arc-2020.09/libgloss/arc/nsim-syscalls.c. While it does work quite OK-insh with nSIM there're some annoying limitations of that implementation. That's due to the usage of "syscall"-based interface very similar to how user-space application calls for Linux kernel services: we put data in registers and execute trap0 (on ARCompat cores) or swi (on ARCv2 cores) instruction to inform the debug host about pending request from the target (see https://github.com/foss-for-synopsys-dwc-arc-processors/newlib/blob/arc-2020.09/libgloss/arc/nsim-syscall.h#L269).

1. It only works with nSIM, but not with real HW target, because real HW doesn't know anything about "tricks" it needs to to when trap0/swi generates an exception.
2. In case of ARC core configured with reduced register file (-rgf_num_regs=16 in ARChitect tool parlance) some of the basic registers are missing (R4-R10 are not implemented in that configuration) so we just cannot pass all the parameters as with complete register file. See [newlib/libgloss] Add support for RF16 configurations #231.

Solution is in use of a so-called "MetaWare Hostlink" interface (used by default by MetaWare tools, thus the name) which is implemented via mail-box in target's memory which is then accessed by debug host either directly in case of simulation or via JTAG in case of execution on FPGA, real silicone or even in RTL simulation (in xCAM or VCS).

Sources of that library are distributed as a part of MetaWare tools (see MetaWare/arc/lib/src/hl/hl_g) but to make sure there're no license problems and we do like how it is done (see https://en.wikipedia.org/wiki/Not_invented_here) it might want to come up with our own implementation as a part of libgloss under BSD-like license and then even substitute with it original MW's implementation to save time on maintenance.

[VVIsaev]

Caught #315 when working on this.

It happens when I try to copy from uncached Metaware Hostlink stat structure to newlib-defined struct stat in fstat() implementation.

[VVIsaev]

Please find PR here: foss-for-synopsys-dwc-arc-processors/newlib#14

Latest Dejagnu test results (Metaware Hostlink vs nSIM GNU IO Hostlink):

• 2 new fails in binutils suite. 2 strip tests failed because Metaware hostlink does not work with stripped binaries.
• 1 more fail for BE ARC newlib - this is because Metaware Extended IO for BE causes nsim to exit (there is code in nsim like if (ext_io hostlink and BE) then exit(FAIL).
• ~500 new libstdc++ tests become supported with Metaware hostlink. So we have ~450 new PASSes and ~50 new FAILs.
• Many test fail in gdb suite. GDB nsim parameters are configured on server, so all tests were run with parameters for GNU IO Hostlink.

[abrodkin]

@shahab-vahedi could you please suggest what can we do to address failures in GDB test suite here?

[claziss]

Closing commit foss-for-synopsys-dwc-arc-processors/newlib@9fd48b9