x86-64: Implement strcat family IFUNC selectors in C

x86-64: Implement strcat family IFUNC selectors in C

Implement strcat family IFUNC selectors in C.

All internal calls within libc.so can use IFUNC on x86-64 since unlike
x86, x86-64 supports PC-relative addressing to access the GOT entry so
that it can call via PLT without using an extra register.  For libc.a,
we can't use IFUNC for functions which are called before IFUNC has been
initialized.  Use IFUNC internally reduces the icache footprint since
libc.so and other codes in the process use the same implementations.
This patch uses IFUNC for strcat family functions within libc.

	* sysdeps/x86_64/multiarch/Makefile (sysdep_routines): Add
	strcat-sse2.
	* sysdeps/x86_64/multiarch/strcat-sse2.S: New file.
	* sysdeps/x86_64/multiarch/strcat.c: Likewise.
	* sysdeps/x86_64/multiarch/strncat.c: Likewise.
	* sysdeps/x86_64/multiarch/strcat.S: Removed.
	* sysdeps/x86_64/multiarch/strncat.S: Likewise.