fix: several compilation issues under LLVM and more

[basilfx]

I am upgrading the U8g2 package version in RIOT-OS. This PR fixes some of the compilation errors I encountered.

I have been able to test this against some displays and board I own. But the warnings below occur in drivers and functionality I cannot test.

Comparison of integers of mixed signs:

This fails for MSP-430 (16-bit targets). Casting rad to u8g2_int_t solves it.

u8g2_arc.c: In function 'u8g2_draw_arc':
u8g2_arc.c:78:16: error: comparison of integer expressions of different signedness: 'u8g2_int_t' {aka 'short int'} and 'unsigned int' [-Werror=sign-compare]
   78 |     else if (d < 2 * (rad - y))
      |                ^

Unused variables (LLVM):

These variables are assigned to, but never read from.

u8x8_d_st7305.c:279:12: error: variable 'x' set but not used [-Werror,-Wunused-but-set-variable]
  uint16_t x;
           ^
u8x8_d_st7305.c:395:12: error: variable 'x' set but not used [-Werror,-Wunused-but-set-variable]
  uint16_t x;
           ^
u8x8_d_st7305.c:609:12: error: variable 'x' set but not used [-Werror,-Wunused-but-set-variable]
  uint16_t x;
           ^
u8x8_d_st7302.c:418:12: error: variable 'x' set but not used [-Werror,-Wunused-but-set-variable]
  uint16_t x;
           ^

Memory alignment requirement (LLVM):

Using memcpy instead resolves this issue.

u8x8_d_sdl_128x64.c:53:13: error: cast from 'uint8_t *' (aka 'unsigned char *') to 'uint32_t *' (aka 'unsigned int *') increases required alignment from 1 to 4 [-Werror,-Wcast-align]
   53 |       ptr = (uint32_t *)(((uint8_t *)(u8g_sdl_screen->pixels)) + offset);
      |             ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Mark variable as unused (LLVM):

max_ptr is only used in assert(..), which may be a no-op if assertions are disabled. Mark it as unused resolves this.

u8g2_ll_hvline.c:79:12: error: unused variable 'max_ptr' [-Werror,-Wunused-variable]
   79 |   uint8_t *max_ptr = u8g2->tile_buf_ptr + u8g2_GetU8x8(u8g2)->display_info->tile_width*u8g2->tile_buf_height*8;
      |            ^~~~~~~
1 error