build: set -O3 and tune WinCE in CI, fix getpart, vtls_scache fallouts

- GHA/windows/WinCE:
  - set `-O3 -DNDEBUG` C flags manually for the CMake mingw32ce build.
    CMake doesn't recognize the platform and fails to add them. To match
    autotools (using `-O2`), and hit similar compiler warnings.
  - enable parallel builds for cmake.
  - tune parallelism for cmake using unity batches.
  - tune parallelism for autotools.

  Follow-up to 2a292c3984 #15975

- tests: fix potentially uninitialized value in `readline()` in
  `getpart.c`. Detected by gcc 4.4.0 `-O2` (Windows CE) jobs:
  ```
  tests/server/getpart.c: In function 'getpart':
  tests/server/getpart.c:298: error: 'datalen' may be used uninitialized in this function
  ```
  Ref: https://github.com/curl/curl/actions/runs/13522595237/job/37785147505?pr=16476#step:11:25
  Follow-up to 592880a3ca

- vtls_scache: rework returning pointer to avoid compiler warning seen
  with `-O3` gcc 4.4.0 builds (Windows CE/schannel):
  ```
  lib/vtls/schannel.c: In function 'schannel_connect_step1':
  lib/vtls/vtls_scache.c:975: error: dereferencing pointer 'old_cred.4474' does break strict-aliasing rules
  lib/vtls/vtls_scache.c:985: error: dereferencing pointer 'old_cred.4474' does break strict-aliasing rules
  lib/vtls/schannel.c:959: note: initialized from here
  ```
  Ref: https://github.com/curl/curl/actions/runs/13523868335/job/37789610845#step:9:25
  Follow-up to fa0ccd9f1f #15774

Closes #16476

tests/server/getpart.c

@@ -130,8 +130,10 @@ static int readline(char **buffer, size_t *bufsize, size_t *length,
   for(;;) {
     int bytestoread = curlx_uztosi(*bufsize - offset);
 
-    if(!fgets(*buffer + offset, bytestoread, stream))
+    if(!fgets(*buffer + offset, bytestoread, stream)) {
+      *length = 0;
       return (offset != 0) ? GPE_OK : GPE_END_OF_FILE;
+    }
 
     *length = offset + line_length(*buffer + offset, bytestoread);
     if(*(*buffer + *length - 1) == '\n')