libc should expose program header flag constants

[vext01]

It'd be useful if libc exposed PF_X, PF_R, PF_W and PF_MASKPROC.

You can currently get PF_{R,W,X} from the elf crate, but arguably libc is a more appropriate home for them and PF_MASKPROC should also be included.

[joshtriplett]

Given that these flags exist in glibc's elf.h, these seem completely reasonable to add to the libc crate. Could you send a pull request adding them?