configure: error: Oops, mp_limb_t is 32 bits, but the assembler code
in this configuration expects 64 bits.
You appear to have set $CFLAGS, perhaps you also need to tell GMP the
intended ABI, see "ABI and ISA" in the manual.

!!! Please attach the following file when filing a report to bugs.gentoo.org:
!!! /var/tmp/portage/dev-libs/gmp-4.2.1-r1/work/gmp-4.2.1/config.log

!!! ERROR: dev-libs/gmp-4.2.1-r1 failed.
Call stack:
  ebuild.sh, line 1614:   Called dyn_compile
  ebuild.sh, line 971:   Called qa_call 'src_compile'
  environment, line 3577:   Called src_compile
  gmp-4.2.1-r1.ebuild, line 54:   Called econf '--localstatedir=/var/state/gmp' '--disable-mpfr' '--disable-mpbsd' '--enable-cxx'
  ebuild.sh, line 577:   Called die

!!! econf failed
!!! If you need support, post the topmost build error, and the call stack if relevant.
!!! A complete build log is located at '/var/tmp/portage/dev-libs/gmp-4.2.1-r1/temp/build.log'.