vendor/awk: upgrade from 20121220 to 20200612

vendor/awk: upgrade from 20121220 to 20200612

Summary of notable changes:

- fixed buffer overflow in error reporting
- OFS value used to rebuild the record was incorrect
- add POSIX standard %a and %A to supported formats
- decrementing NF did not change $0
- unary plus on a string constant returned the string
- avoid undefined behaviour when using ctype(3) functions in relex()
- make getline handle numeric strings
- added support for POSIX-standard interval expressions in regexps
- use of random(3) as the random number generator
- support POSIX-specified C-style escape sequences "\a" and "\v" in
  command line arguments and regular expressions
- fix various array / memory overruns
- import code from NetBSD awk that implements RS as a regular expression
- add an optimization for string concatenation
- fix a bug whereby a{0,3} could match four a's
- input/output errors on closing files are now fatal instead of mere
  warnings
- if POSIXLY_CORRECT is set in the environment, then sub and gsub use
  POSIX rules for multiple backslashes
- it's no longer necessary to use the -y flag to bison
- backslash continuation inside strings removes the newline
- avoid any spurious errors left over from previous calls

For details, see FIXES, or commit history at
https://github.com/onetrueawk/awk/commits/master