Create a lexer for GDB debug session

[mithro]

GDB is a common C/C++/etc debugger. The output often appears in low level help documentation. It would be awesome if it was supported by Pygments!

Here is an example GDB session;

$ riscv64-unknown-elf-gdb riscv-blink.elf -ex 'target remote localhost:1234'
GNU gdb (GDB) 8.2.90.20190228-git
Copyright (C) 2019 Free Software Foundation, Inc.
License GPLv3+: GNU GPL version 3 or later <http://gnu.org/licenses/gpl.html>
This is free software: you are free to change and redistribute it.
There is NO WARRANTY, to the extent permitted by law.
Type "show copying" and "show warranty" for details.
This GDB was configured as "--host=x86_64-w64-mingw32 --target=riscv64-unknown-elf".
Type "show configuration" for configuration details.
For bug reporting instructions, please see:
<http://www.gnu.org/software/gdb/bugs/>.
Find the GDB manual and other documentation resources online at:
    <http://www.gnu.org/software/gdb/documentation/>.

For help, type "help".
Type "apropos word" to search for commands related to "word"...
Reading symbols from .\riscv-blink.elf...
Remote debugging using localhost:1234
csr_writel (addr=3758106660, value=1) at ./include/hw/common.h:41
41              *((volatile uint32_t *)addr) = value;
(gdb)
(gdb) bt
#0  0x2004014c in csr_readl (addr=3758106664) at ./include/hw/common.h:46
#1  timer0_value_read () at ./include/generated/csr.h:242
#2  0x200401dc in msleep (ms=ms@entry=80) at ./include/hw/common.h:41
#3  0x20040074 in main () at ./src/main.c:45
(gdb)

[mithro]

I'll probably take a stab at making this lexer.