Cross-compilable

Detail:
* Makefile changed to use CModule shared makefile in order to utilise its new
  ASM2TXT feature to simplify autogeneration of the exported FileCoreErr.h.
* Can now build a HAL-agnostic variant, for example in a disc build.
* .gitignore moved to top level and populated with all temporary file paths.
* Enable GitLab CI and address issues identified.

Admin:
  Requires RiscOS/BuildSys!3
!NoTag
13 jobs for !1 with CrossCompilationSupport in 21 seconds (queued for 27 seconds)
latest detached
Status Job ID Name Coverage
  Static Analysis
passed #56469
cross
copyright

00:00:00

passed #56470
cross
cppcheck

00:00:04

passed #56460
cross
gitattributes

00:00:01

passed #56461
cross
gitignore

00:00:01

passed #56464
cross
head_log

00:00:00

passed #56467
cross
head_whitesp

00:00:00

passed #56462
cross
license

00:00:00

passed #56466
cross
makefile

00:00:00

passed #56465
cross
merge_log

00:00:00

passed #56468
cross
merge_whitesp

00:00:00

passed #56463
cross
versionnum

00:00:00

 
  Softload
passed #56471
cross
softload

00:00:05

 
  Cleanup
passed #56472
cross
cleanup

00:00:01