powerpc: Use builtin functions for fls()/__fls()/fls64()

With the fls() functions as defined in arch/powerpc/include/asm/bitops.h
GCC will not optimise the code in case of constant parameter.

This patch replaces __fls() by the builtin function, and modifies
fls() and fls64() to use builtins instead of inline assembly

For non constant calls, the generated code is doing the same:

int testfls(unsigned int x)
{
	return fls(x);
}

unsigned long test__fls(unsigned long x)
{
	return __fls(x);
}

int testfls64(__u64 x)
{
	return fls64(x);
}

On PPC32, before the patch:
00000064 <testfls>:
  64:	7c 63 00 34 	cntlzw  r3,r3
  68:	20 63 00 20 	subfic  r3,r3,32
  6c:	4e 80 00 20 	blr

00000070 <test__fls>:
  70:	7c 63 00 34 	cntlzw  r3,r3
  74:	20 63 00 1f 	subfic  r3,r3,31
  78:	4e 80 00 20 	blr

0000007c <testfls64>:
  7c:	2c 03 00 00 	cmpwi   r3,0
  80:	40 82 00 10 	bne     90 <testfls64+0x14>
  84:	7c 83 00 34 	cntlzw  r3,r4
  88:	20 63 00 20 	subfic  r3,r3,32
  8c:	4e 80 00 20 	blr
  90:	7c 63 00 34 	cntlzw  r3,r3
  94:	20 63 00 40 	subfic  r3,r3,64
  98:	4e 80 00 20 	blr

On PPC32, after the patch:
00000054 <testfls>:
  54:	7c 63 00 34 	cntlzw  r3,r3
  58:	20 63 00 20 	subfic  r3,r3,32
  5c:	4e 80 00 20 	blr

00000060 <test__fls>:
  60:	7c 63 00 34 	cntlzw  r3,r3
  64:	20 63 00 1f 	subfic  r3,r3,31
  68:	4e 80 00 20 	blr

0000006c <testfls64>:
  6c:	2c 03 00 00 	cmpwi   r3,0
  70:	41 82 00 10 	beq     80 <testfls64+0x14>
  74:	7c 63 00 34 	cntlzw  r3,r3
  78:	20 63 00 40 	subfic  r3,r3,64
  7c:	4e 80 00 20 	blr
  80:	7c 83 00 34 	cntlzw  r3,r4
  84:	20 63 00 40 	subfic  r3,r3,32
  88:	4e 80 00 20 	blr

On PPC64, before the patch:
00000000000000a0 <.testfls>:
  a0:	7c 63 00 34 	cntlzw  r3,r3
  a4:	20 63 00 20 	subfic  r3,r3,32
  a8:	7c 63 07 b4 	extsw   r3,r3
  ac:	4e 80 00 20 	blr

00000000000000b0 <.test__fls>:
  b0:	7c 63 00 74 	cntlzd  r3,r3
  b4:	20 63 00 3f 	subfic  r3,r3,63
  b8:	7c 63 07 b4 	extsw   r3,r3
  bc:	4e 80 00 20 	blr

00000000000000c0 <.testfls64>:
  c0:	7c 63 00 74 	cntlzd  r3,r3
  c4:	20 63 00 40 	subfic  r3,r3,64
  c8:	7c 63 07 b4 	extsw   r3,r3
  cc:	4e 80 00 20 	blr

On PPC64, after the patch:
0000000000000090 <.testfls>:
  90:	7c 63 00 34 	cntlzw  r3,r3
  94:	20 63 00 20 	subfic  r3,r3,32
  98:	7c 63 07 b4 	extsw   r3,r3
  9c:	4e 80 00 20 	blr

00000000000000a0 <.test__fls>:
  a0:	7c 63 00 74 	cntlzd  r3,r3
  a4:	20 63 00 3f 	subfic  r3,r3,63
  a8:	4e 80 00 20 	blr
  ac:	60 00 00 00 	nop

00000000000000b0 <.testfls64>:
  b0:	7c 63 00 74 	cntlzd  r3,r3
  b4:	20 63 00 40 	subfic  r3,r3,64
  b8:	7c 63 07 b4 	extsw   r3,r3
  bc:	4e 80 00 20 	blr

Those builtins have been in GCC since at least 3.4.6 (see
https://gcc.gnu.org/onlinedocs/gcc-3.4.6/gcc/Other-Builtins.html

 )

Signed-off-by: Christophe Leroy <christophe.leroy@c-s.fr>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>