runtime: fix nacl build

The NaCl "system calls" were assumed to have a compatible
return convention with the C compiler, and we were using
tail jumps to those functions. Don't do that anymore.

Correct mistake introduced in newstackcall duringconversion
from (SP) to (FP) notation. (Actually this fix, in asm_amd64p32.s,
slipped into the C compiler change, but update the name to
match what go vet wants.)

Correct computation of caller stack pointer in morestack:
on amd64p32, the saved PC is the size of a uintreg, not uintptr.
This may not matter, since it's been like this for a while,
but uintreg is the correct one. (And on non-NaCl they are the same.)

This will allow the NaCl build to get much farther.
It will probably still not work completely.
There's a bug in 6l that needs fixing too.

TBR=minux
CC=golang-codereviews
https://codereview.appspot.com/134990043
cmd/5l, cmd/6l, cmd/8l: fix nacl binary corruption bug

NaCl requires the addition of a 32-byte "halt sled" at the end
of the text segment. This means that segtext.len is actually
32 bytes shorter than reality. The computation of the file offset
of the end of the data segment did not take this 32 bytes into
account, so if len and len+32 rounded up (by 64k) to different
values, the symbol table overwrote the last page of the data
segment.

The last page of the data segment is usually the C .string
symbols, which contain the strings used in error prints
by the runtime. So when this happens, your program
probably crashes, and then when it does, you get binary
garbage instead of all the usual prints.

The chance of hitting this with a randomly sized text segment
is 32 in 65536, or 1 in 2048.

If you add or remove ANY code while trying to debug this
problem, you're overwhelmingly likely to bump the text
segment one way or the other and make the bug disappear.

Correct all the computations to use segdata.fileoff+segdata.filelen
instead of trying to rederive segdata.fileoff.

This fixes the failure during the nacl/amd64p32 build.

TBR=iant
CC=golang-codereviews
https://codereview.appspot.com/135050043