toolchain-funcs.eclass: Add tc-is-lto function

Add a function to check whether the C compiler is using LTO.
To determine this, we compile a dummy source unit.  In the GCC case,
we check whether the resulting object file contains ".gnu.lto*"
sections.  In the clang case, we check whether a valid LLVM bytecode
file was output rather than a regular object.

The goal of this change is to reduce the amount of USE=lto abuse,
and have a consistent cross-package way of enabling LTO via setting
appropriate CFLAGS and CXXFLAGS.

Closes: https://github.com/gentoo/gentoo/pull/34470
Signed-off-by: Michał Górny <mgorny@gentoo.org>

eclass/toolchain-funcs.eclass

@@ -1230,4 +1230,25 @@ tc-get-build-ptr-size() {
 		die "Could not determine CBUILD pointer size"
 }
 
+# @FUNCTION: tc-is-lto
+# @RETURN: Shell true if we are using LTO, shell false otherwise
+tc-is-lto() {
+	local f="${T}/test-lto.o"
+
+	case $(tc-get-compiler-type) in
+		clang)
+			$(tc-getCC) ${CFLAGS} -c -o "${f}" -x c - <<<"" || die
+			# If LTO is used, clang will output bytecode and llvm-bcanalyzer
+			# will run successfully.  Otherwise, it will output plain object
+			# file and llvm-bcanalyzer will exit with error.
+			llvm-bcanalyzer "${f}" &>/dev/null && return 0
+			;;
+		gcc)
+			$(tc-getCC) ${CFLAGS} -c -o "${f}" -x c - <<<"" || die
+			[[ $($(tc-getREADELF) -S "${f}") == *.gnu.lto* ]] && return 0
+			;;
+	esac
+	return 1
+}
+
 fi