cmd/compile: build compiler with PGO

cmd/compile: build compiler with PGO

Build the compiler with PGO. As go build -pgo=auto is enabled by
default, we just need to store a profile in the compiler's
directory.

The profile is collected from building all std and cmd packages on
Linux/AMD64 machine, using profile.sh.

This improves the compiler speed. On Linux/AMD64,
name        old time/op       new time/op       delta
Template          138ms ± 5%        136ms ± 4%  -1.44%  (p=0.005 n=36+39)
Unicode           147ms ± 4%        140ms ± 4%  -4.99%  (p=0.000 n=40+39)
GoTypes           780ms ± 3%        778ms ± 4%    ~     (p=0.172 n=39+39)
Compiler          105ms ± 5%         99ms ± 7%  -5.64%  (p=0.000 n=40+40)
SSA               5.83s ± 6%        5.80s ± 6%    ~     (p=0.556 n=40+40)
Flate            89.0ms ± 5%       87.0ms ± 6%  -2.18%  (p=0.000 n=40+40)
GoParser          172ms ± 4%        167ms ± 4%  -2.72%  (p=0.000 n=39+40)
Reflect           333ms ± 4%        333ms ± 3%    ~     (p=0.426 n=40+39)
Tar               128ms ± 4%        126ms ± 4%  -1.82%  (p=0.000 n=39+39)
XML               173ms ± 4%        170ms ± 4%  -1.39%  (p=0.000 n=39+40)
[Geo mean]        253ms             248ms       -2.13%

The profile is pretty transferable. Using the same profile, we
see a bigger win on Darwin/ARM64,
name        old time/op       new time/op       delta
Template         71.0ms ± 2%       68.3ms ± 2%  -3.90%  (p=0.000 n=20+20)
Unicode          71.8ms ± 2%       66.8ms ± 2%  -6.90%  (p=0.000 n=20+20)
GoTypes           444ms ± 1%        428ms ± 1%  -3.53%  (p=0.000 n=19+20)
Compiler         48.9ms ± 3%       45.6ms ± 3%  -6.81%  (p=0.000 n=20+20)
SSA               3.25s ± 2%        3.09s ± 1%  -5.03%  (p=0.000 n=19+20)
Flate            44.0ms ± 2%       42.3ms ± 2%  -3.72%  (p=0.000 n=19+20)
GoParser         76.7ms ± 1%       73.5ms ± 1%  -4.15%  (p=0.000 n=18+19)
Reflect           172ms ± 1%        165ms ± 1%  -4.13%  (p=0.000 n=20+19)
Tar              63.1ms ± 1%       60.4ms ± 2%  -4.24%  (p=0.000 n=19+20)
XML              83.2ms ± 2%       79.2ms ± 2%  -4.79%  (p=0.000 n=20+20)
[Geo mean]        127ms             121ms       -4.73%

Change-Id: I44735b3f7fd6903efbbe6b19c05dee874bea4c89
Reviewed-on: https://go-review.googlesource.com/c/go/+/451292
Run-TryBot: Cherry Mui <cherryyz@google.com>
Reviewed-by: Michael Pratt <mpratt@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>