build: add support for riscv64 architecture

[kxxt]

This PR introduces basic support for riscv64 in the build system to make srsRAN build-able on riscv64 architecture.

There are some test failures on riscv64 architecture(I ran the tests with 23.11):

60% tests passed, 608 tests failed out of 1523

test.log

[CLAassistant]

Thank you for your submission! We really appreciate it. Like many open source projects, we ask that you sign our Contributor License Agreement before we can accept your contribution.
_{You have signed the CLA already but the status is still pending? Let us recheck it.}