toolchain.eclass: fix tc-enables-* with -ggdb3

toolchain.eclass: fix tc-enables-* with -ggdb3

-ggdb3 builds debug information for macros into binaries. For this reason,
the C preprocessor includes all #defines as-is in the output. The check
in tc-enables-* expects only a fixed output of "true", which fails in this
scenario.

To fix this, `grep` is used to look specifically for "true" in the output
from the preprocessor.

Closes: https://github.com/gentoo/gentoo/pull/5359
Signed-off-by: Sergei Trofimovich <slyfox@gentoo.org>
Signed-off-by: Matthias Maier <tamiko@gentoo.org>