Build with PGO - it's more than 5% faster

We can probably get a more realistic PGO profile than just running the
microbenchmarks, but it'll do.

goawk=go1.21.0 with PGO
orig =go1.21.0 without PGO

Test                         |    goawk |     orig |      awk
---------------------------- | -------- | -------- | --------
tt.01 (print)                |     3.02 |     2.76 |     1.00
tt.02 (print NR NF)          |     2.07 |     1.95 |     1.00
tt.02a (print length)        |     1.85 |     1.80 |     1.00
tt.03 (sum length)           |     1.53 |     1.47 |     1.00
tt.03a (sum field)           |     1.47 |     1.44 |     1.00
tt.04 (printf fields)        |     1.36 |     1.21 |     1.00
tt.05 (concat fields)        |     1.54 |     1.48 |     1.00
tt.06 (count lengths)        |     1.64 |     1.55 |     1.00
tt.07 (even fields)          |     1.49 |     1.43 |     1.00
tt.08 (even lengths)         |     2.54 |     2.60 |     1.00
tt.08z (regex simple)        |     3.73 |     3.71 |     1.00
tt.09 (regex starts with)    |     2.87 |     2.83 |     1.00
tt.10 (regex ends with)      |     0.47 |     0.45 |     1.00
tt.10a (regex ends with var) |     0.45 |     0.44 |     1.00
tt.11 (substr)               |     3.35 |     3.16 |     1.00
tt.12 (update fields)        |     1.53 |     1.51 |     1.00
tt.13 (array ops)            |     4.84 |     4.28 |     1.00
tt.13a (array printf)        |     3.03 |     2.83 |     1.00
tt.14 (function call)        |     1.73 |     1.50 |     1.00
tt.15 (format lines)         |     1.00 |     0.97 |     1.00
tt.16 (count words)          |     1.92 |     1.77 |     1.00
tt.big (complex program)     |     2.29 |     2.17 |     1.00
tt.x1 (mandelbrot)           |     2.68 |     2.67 |     1.00
tt.x2 (sum loop)             |     2.41 |     2.19 |     1.00
---------------------------- | -------- | -------- | --------
**Geo mean**                 | **1.86** | **1.76** | **1.00**

Author: benhoyt

.gitignore

@@ -13,3 +13,4 @@ goawk_*.tar.gz
 interp/testdata
 scripts/csvbench/bin
 scripts/csvbench/count.csv
+/default.pgo

scripts/make_binaries.sh

@@ -1,5 +1,8 @@
 #!/bin/sh
 
+# Create PGO profile
+go test -run=^$ -bench=. -cpuprofile=default.pgo ./interp
+
 go build
 VERSION="$(./goawk -version)"