Auto merge of #31388 - gmbonnet:compiler-rt-werror, r=alexcrichton

Without this patch, `compiler-rt` fails to build when the `CFLAGS` environment variable contains a `-Werror=*` flag (for example `-Werror=format-security`).

The build system was removing only the `-Werror` part from the flag, thus passing an unrecognized `=*` (for example `=format-security`) argument to gcc.

Author: bors

mk/rt.mk

@@ -245,7 +245,7 @@ COMPRT_AR_$(1) := $$(AR_$(1))
 # We chomp -Werror here because GCC warns about the type signature of
 # builtins not matching its own and the build fails. It's a bit hacky,
 # but what can we do, we're building libclang-rt using GCC ......
-COMPRT_CFLAGS_$(1) := $$(subst -Werror,,$$(CFG_GCCISH_CFLAGS_$(1))) -std=c99
+COMPRT_CFLAGS_$(1) := $$(filter-out -Werror -Werror=*,$$(CFG_GCCISH_CFLAGS_$(1))) -std=c99
 
 # FreeBSD Clang's packaging is problematic; it doesn't copy unwind.h to
 # the standard include directory. This should really be in our changes to