[libc++] Add GitHub usernames for Linaro managed bots (#133120)

This isn't the whole team, but enough that one of us will see it and
make sure the right person sees it.

Noted explicitly that Picolibc is also our responsiblity as it's not
always clear that that is tested on Arm.

libcxx/utils/ci/BOT_OWNERS.txt

@@ -5,12 +5,13 @@ cannot be figured out by the community alone.
 The list is sorted by surname and formatted to allow easy grepping and
 beautification by scripts. The fields are: name (N), email (E), web-address
 (W), PGP key ID and fingerprint (P), description (D), snail-mail address
-(S), Phabricator handle (H) and IRC handle (I). Each entry should contain at
-least the (N), (E) and (D) fields.
+(S), Phabricator handle (H), IRC handle (I) and GitHub username(s) (G).
+Each entry should contain at least the (N), (E) and (D) fields.
 
 N: Linaro Toolchain Working Group
 E: linaro-toolchain@lists.linaro.org
-D: Arm platforms
+D: Arm platforms and picolibc
+G: DavidSpickett, omjavaid, ceseo
 
 N: LLVM on Power
 E: powerllvm@ca.ibm.com