libgcc_s: export __extendhftf2

We've compiled it since the LLVM 13 import in 2021, but for some reason
never exported it.  A user of CheriBSD recently caused Morello LLVM to
emit a reference to it so finish the job and export it.

Reviewed by:	dim, emaste
Fixes:		6e75b2fbf9 ("Merge llvm-project release/13.x llvmorg-13.0.0-rc1-97-g23ba3732246a")
MFC after:	1 week
Sponsored by:	DARPA, AFRL
See also:	https://github.com/CTSRD-CHERI/cheribsd/issues/2614
Differential Revision:	https://reviews.freebsd.org/D56310

lib/libgcc_s/Symbol.map

@@ -198,6 +198,7 @@ GCC_4.6.0 {
 	__divtf3;
 	__eqtf2;
 	__extenddftf2;
+	__extendhftf2;
 	__extendsftf2;
 	__fixtfdi;
 	__fixtfsi;