Simplify build id generation

[dmitrys99]

I'm building SBCL on Windows under MinGW64.
My computer's name contains non-Ascii character (namely "Домашний", which is in Russian).
User name has non-Ascii characters as well.

So build fails.
Below is a part of output.

` ...
obj/from-xc/src/compiler/late-proclaim.lisp-obj
obj/from-xc/src/code/defboot.lisp-obj
obj/from-xc/src/code/setf.lisp-obj
obj/from-xc/src/code/macros.lisp-obj
obj/from-xc/src/code/loop.lisp-obj
obj/from-xc/src/code/late-format.lisp-obj
obj/from-xc/src/code/sxhash.lisp-obj
obj/from-xc/src/code/late-globaldb.lisp-obj
obj/from-xc/src/pcl/walk.lisp-obj
; SB!Loader: (274+725+11193+163+14252) structs/consts/funs/methods/other
[building initial core file in "output/cold-sbcl.core":
WARNING: Found non-STANDARD-CHAR in "▒▒▒▒▒▒▒▒-Соло-2018-04-09-13-30-31"
While evaluating the form starting at line 16, column 0
of #P"C:/dev/sbcl-src/sbcl-1.4.6/make-genesis-2.lisp":

debugger invoked on a TYPE-ERROR in thread
#<THREAD "main thread" RUNNING {10027900C3}>:
The value
#\CYRILLIC_CAPITAL_LETTER_DE
is not of type
STANDARD-CHAR
when binding CHARACTER

Type HELP for debugger help, or (SB-EXT:EXIT) to exit from SBCL.

restarts (invokable by number or by possibly-abbreviated name):
0: [RETRY ] Retry EVAL of current toplevel form.
1: [CONTINUE] Ignore error and continue loading file "C:\dev\sbcl-src\sbcl-1.4.6\make-genesis-2.lisp".
2: [ABORT ] Abort loading file "C:\dev\sbcl-src\sbcl-1.4.6\make-genesis-2.lisp".
3: Abort building SBCL.
4: Exit debugger, returning to top level.

(SB!XC:CHAR-CODE #\CYRILLIC_CAPITAL_LETTER_DE) [external]
source: (DEFUN SB!XC:CHAR-CODE (CHARACTER)
(DECLARE (TYPE STANDARD-CHAR CHARACTER))
(IF (CHAR= CHARACTER #\Newline)
10
(+ (POSITION CHARACTER ASCII-STANDARD-CHARS) 32)))
0] //testing for consistency of first and second GENESIS passes
diff: output/genesis-2: No such file or directory
error: header files do not match between first and second GENESIS

real 2m4.223s
user 0m0.030s
sys 0m0.151s`

There are several problems.

1. hostname returns name of the host with encoding CP1251 (in my case)
2. id -un returns logged user name with encoding UTF-8
3. genesis expects build id is simple-string

So constructed build id malformed from the very beginning.

Proposed solution:

The main purpose of build id to discourage people from mismatching sbcl and *.core files.
If we eliminate hostname and change username to user id (which is done by id -u instead if id -un, we still get good uniqueness without mentioned problems.

[equwal]

Patch submitted to the mailing list.