illumos gate

As of GNU binutils 2.37, the assembler has split the 'sysexit' instruction
into 'sysexitl' and 'sysexitq'. Using a plain 'sysexit' as kpti_trampolines.s
does is interpreted as 'sysexitl' but comes with a warning about the
assumption being made. By default, the assembler wrapper - aw -
treats all errors as fatal.

The plain 'sysexit' in this file cannot be changed to 'sysexitl' since older
versions of GNU binutils do not understand that.

I tested building this change on an OmniOS system with binutils 2.36.1 and then with binutils 2.37. Both succeeded and wsdiff showed no differences between these and a build without the change.
Disassembling the updated function shows the expected instructions:

build:ig_13958_kpti.s:ig_13958_kpti.s% dis -F tr_sysexit proto/root_i386-nd/platform/i86pc/kernel/amd64/unix
disassembly for proto/root_i386-nd/platform/i86pc/kernel/amd64/unix

tr_sysexit()
    tr_sysexit:      9c                 pushfq
    tr_sysexit+0x1:  48 83 3c 25 00 00  cmpq   $0x1,0xfffffffffbc00000  <kpti_enable>
                     c0 fb 01
    tr_sysexit+0xa:  75 3c              jne    +0x3c    <tr_sysexit+0x48>
    tr_sysexit+0xc:  9d                 popfq
    tr_sysexit+0xd:  65 4c 89 2c 25 58  movq   %r13,%gs:0x1e58
                     1e 00 00
    tr_sysexit+0x16: 65 4c 8b 2c 25 b0  movq   %gs:0x1eb0,%r13
                     1e 00 00
    tr_sysexit+0x1f: 41 0f 22 dd        movq   %r13,%cr3
    tr_sysexit+0x23: 65 4c 8b 2c 25 58  movq   %gs:0x1e58,%r13
                     1e 00 00
    tr_sysexit+0x2c: 65 48 c7 04 25 58  movq   $0x0,%gs:0x1e58
                     1e 00 00 00 00 00
                     00
    tr_sysexit+0x39: 65 48 c7 04 25 50  movq   $0x0,%gs:0x1e50
                     1e 00 00 00 00 00
                     00
    tr_sysexit+0x46: eb 01              jmp    +0x1     <tr_sysexit+0x49>
    tr_sysexit+0x48: 9d                 popfq
    tr_sysexit+0x49: 0f 01 f8           swapgs
    tr_sysexit+0x4c: fb                 sti
    tr_sysexit+0x4d: 0f 35              sysexit