devel/freebsd-gcc*: also build libgcov.a

[siva]

Summary

This fixes the FreeBSD LINT[0] kernel build that includes GCOV[1].

Other changes:

• explicit --disable-multilib is required to prevent the libgcc/libgcov.a submake build from building a multilib libgcov.a
• armv7 requires additional _FOR_TARGET make args because it erroneously looks for 'armv7-' prefixed binutils
• these changes require a workaround to ensure that build targets are processed sequentially, because at higher core counts, configure-target-libgcc fails when it can't find a prebuilt xgcc binary.

[0] https://cgit.freebsd.org/src/tree/sys/amd64/conf/NOTES?id=886164895f3f4805dc11171ac7980acaccf6ec77#n10
[1] https://cgit.freebsd.org/src/tree/sys/conf/kern.pre.mk?id=886164895f3f4805dc11171ac7980acaccf6ec77#n104

[jhb]

To date we have purposely excluded libgcc from these ports and instead used libcompiler_rt (sometimes enabling other builtins only used by GCC in libgcc.a in base for example). I don't want these toolchains to ship libgcc to avoid linking with the wrong one, nor should they ship unwind.h, etc. I know that the old GCOV code was not built with these ports (bug the kernel was built with lang/gcc at the time). Does libgcov.a require libgcc.a from GCC or do you just need to use this to get it built and we could restrict the install step to only installing libgcov.a?

[siva]

we could restrict the install step to only installing libgcov.a?

Thanks for the suggestion, applied this feedback in v2 and restricted
the build to only include libgcov.a. This is good enough and
the GCOV build continues to pass with v2 of this patch.