[Bug target/84475] pthread doesn't define _REENTRANT in preprocessor on riscv-linux

[joseph at codesourcery dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=84475

--- Comment #3 from joseph at codesourcery dot com <joseph at codesourcery dot 
com> ---
Note that _REENTRANT is a no-op with glibc 2.25 or later unless you 
specifically select an old standard version (in which case it's an alias 
for _POSIX_C_SOURCE=199506L).